The Department of Energy's goals for concentrated solar power (CSP) systems include increasing the use of CSP in the United States, making CSP competitive in the intermediate power market by 2015 and developing advanced technologies that will enable CSP to be competitive in the baseload market by 2020. CSP traditionally uses concentrated solar energy to drive heat engines, such as the Rankine Cycle, Brayton Cycle and the Stirling Cycle, producing electricity. Traditional CSP systems have a limited capacity factor in the range of 20-25% since they generate electricity only when direct sunlight is available. This limits the ability of CSP to be used for baseload power generation. The capacity factor for CSP systems can be increased by storing energy received at the collector in a form that can be converted to electricity at later times. Storing solar energy as sensible or latent heat is one option.
Another promising option is converting solar energy to chemical energy in a thermochemical reaction. In this case the energy is stored in the chemical product and this chemical energy can be released as heat for power generation at a later time in an exothermic chemical reaction. An advantage of this closed cycle approach is that the chemical products can be stored at ambient temperatures simplifying transport and storage requirements, and enabling longer term storage without energy degradation. An open cycle approach to solar-thermal to chemical energy conversion is also possible. For instance, solar reforming can be used to upgrade the chemical energy content of a fuel stream such as methane from natural gas or biomass when the sun is available, and this upgraded fuel stream can be used to generate electricity using a heat engine or fuel cell. When direct sunlight is not available, the fossil fuel or biomass energy source can still be used to generate electricity using the same infrastructure. Such a system can be used for baseload power generation and can take advantage of solar energy at high conversion efficiencies when available. Thermochemical energy conversions may also enable CSP systems to co-generate electricity and fuels for other markets (e.g., transportation). Methanol and long-chain hydrocarbons are examples of products that can be produced from syngas, which is a product of a reforming reactor.